Amplification and FeedbackAmplifiers• Make a signal (voltage or current) bigger • Voltage amplifier: Voltage out = Voltage in x G (gain)• Small signals: Sensor outputs, radio reception, microelectronics• Big signals: Provide mechanical energy (motors, speakers), radio transmission, heatingThe ideal amplifierGVin Vout = G x VinAmplification devices• Mechanism in which a small signal controls a large signal• Water analogy: turning the faucet (small signal) controls large flow of water (big signal)Vacuum tubes• Electrons “boil” off of heated cathode• Voltage on grid(s) control current reaching anodeAnodeCathodeHeaterelectronsTransistor• Invented 1947• Made of semiconductors – silicon, germanium, gallium arsenide• Layered structure – creates junctionsThe first transistor –Bell Labs, 1947Transistors• Voltage between base and emitter controls current between collector and emitter• Types: bipolar (NPN, PNP), JFET, MOSFET~ 1 µmApproximate transistor behavior –the Ebers-Moll equations• Explicit temperature dependence• Parameters (αF, αR, IES, ICS) dependent upon manufacturing details, temperature, etc.Simple device – complicated behavior• The physical laws which describe electronic behavior are not simple• Want to make electronic components “act” simple to make them easier to apply• Need to build a complicated device to get simple behavior!How to make electronics “act” simple? • Emphasize use of components which have close to ideal behavior– Resistors, capacitors• Use circuits which are inherently self-compensating for nonideal behavior– Ex: voltage dividerR1R2VoutVinModern operational amplifiersLT1006 LMH6642~ 1 mmWhat’s a good “building block” for amplification?• One option: Fixed gains (10, 100, etc)– Instrumentation amplifier• Another option: Make G…really big, then “throw away” gain you don’t needGVin Vout = G x VinOperational amplifiers+-V+V-Vout=G x (V+-V-)• G very large (AD820: G ~= 1 million (DC, low load))• Inputs draw little current (AD820: ~10 pA)Feedback amplifier• Feedback: output “feeds back” to input• Op amps are always used with feedback• How does this work?+-VinVoutR1R2NoninvertingFeedbackAmplifierGRationale for use of feedback amplifiers• Amplifier performance is dictated largely by the behavior of simple passive components (e.g. resistors, capacitors)• Pioneers: H. S. Black, H. Nyquist, H. W. Bode, Ball Laboratories, 1920’s“Black’s patent application was delayed for more than nine years in part because the concept was so contrary to established beliefs that the Patent Office initially did not believe it would work. They treated the application in the same manner as one for a perpetual motion machine.” -- W. M. Siebert, Signals and SystemsBasic op amp configurations• Noninverting• Inverting+-VinVout = -(R2/R1) VinR1R2+-VinVout = Vin (R1+R2)/R2 R1R2Quick analysis of op amp circuits•Because G is so large, the difference between V+and V-must be very small• To a good approximation V+=V-• Ex: V- is the output of a voltage divider: V- = Vout * R2/(R1+R2). V+ = V-, so Vout = V+ (R1+R2)/R2.+-VinVoutR1R2Other op amp configurations• Endless variety of circuits for performing different functions: filtering (high pass, low pass, band pass, band reject), integration, differentiation, calculating logarithms, square roots, etc.• Use different feedback components and interconnectionsEx: Sallen-key lowpass filterBuffer amplifier• Gain of 1 – why use it at all?+-37462+5VVinVoutOutput impedance• Ideal voltage source: Voltage independent of load• Real voltage source: Acts like an ideal voltage source in series with a resistor • Small resistance = “low output impedance”+-IdealReal+-RRLVoutEx: Voltage dividerR1R2VoutVinRL+-37462+5VVoutR1R2VinRLNo buffer amplifier: Vout depends on RLWith buffer amplifier: Vout Independent of RLBuffer amplifier – gain of 1• Buffer amplifier decreases the output impedance of a signal• Makes it easier to transmit signals between parts of a circuit – enables modular construction+-37462+5VVinVoutOp amp packaging• Generally 1, 2, or 4 to a package• Standard pinoutsSingle DualQuadOther op amp parameters• Speed (slew rate, bandwidth) – what type of signal can be amplified– DC, audio, video• Precision (input offset voltage) – how closely does the op amp equalize its inputs• Power (supply current, output current) – how much power does the amplifier require, and how much power can it deliver to a loadInput and output range• Op amp can only amplify signals within a range set by its power supply – can limit applications• “Single supply” op amps have input and output voltage range extending to the negative supplyPowerSupply-5VInputrangeOutputrange5VPowerSupply0VInputrangeOutputrange5VDual supply Single
View Full Document
Unlocking...